Over the past three decades, due to increasing advances in and general acceptance of color television, monochrome CRT design and development efforts have been directed almost exclusively toward use in computer monitors, projection television receivers, and very high resolution medical applications. Color CRTs employ a plurality of electron beams, each beam providing a primary color, while a monochrome CRT makes use of a single electron beam, where in both cases the electron beam, or beams, are horizontally swept across the CRT's display screen in a raster-like manner. The most important operating criteria for a monochrome CRT display are typically high video image resolution and brightness. Unfortunately, these two operating criteria are inter-related such that improvement in one performance parameter generally has an adverse effect on the other.
Other important monochrome CRT performance criteria relate to the magnetic deflection yoke scan frequency and cathode electron emission density. A high deflection yoke scan frequency is generally required for high video image resolution, while high electron emission density is required to provide a high level of video image brightness and resolution. Increased deflection yoke scan frequencies not only require increased input power, but also substantially increase the cost of the CRT deflection yoke. To maintain a high level of video image brightness without diminishing image resolution, a dispenser cathode is sometimes incorporated in the CRT. Dispenser cathodes are expensive, however, costing approximately 50 times more than a conventional oxide cathode.
Video image brightness is also a concern in projection television receivers. A conventional electrostatic focusing electron gun cannot meet both the beam spot size (resolution) and brightness operating criteria because of the large size of a projection television receiver display. A combined electrostatic and magnetic focusing arrangement is typically employed in a high definition television (HDTV) system, which increases the complexity and cost over that of a conventional electron gun and deflection yoke system. In addition, in a high resolution electron gun due to a high video drive frequency, the capacitance of the cathode has to be reduced to 2 pf, or less, which requires a specialized design of increased cost. In the past, multi-beam electron gun designs such as the one described in the May, 1970 edition of Information Display in an article entitled "A Multi-Beam CRT" by D. L. Say, have been too complicated to be practical and have thus not been incorporated in a commercial product.
Referring to FIG. 1, there is shown an isometric view partially in phantom of a typical prior art single beam electron gun 10 for use in a monochrome CRT. A sectional view of the electron gun 10 shown in FIG. 1 taken along site line 2--2 therein is shown in FIG. 2. Electron gun 10 includes a heated cathode 12 which provides energetic electrons in the general direction of a G1 control grid 16 having an aperture 16a through which the energetic electrons travel. Electron gun 10 further includes a G2 screen grid 18 aligned with the G1 control grid 16 and also including a center aperture 18a through which the energetic electrons travel. The G1 control and G2 screen grids 16, 18 provide a beam forming region (BFR) 28 in electron gun 10 for forming the energetic electrons emitted by cathode 12 into an electron beam 14 shown in dotted-line form in the figures. Electron gun 10 further includes a G3 grid 20 and a G4 grid 22 each generally cylindrical and aligned along the electron gun's longitudinal axis Z--Z'. The G3 grid 20 includes an aperture 20a in an end thereof for passing the electron beam 14 toward the cylindrical, open G4 grid 22. The combination of the G3 and G4 grids 20, 22 forms a high voltage focusing lens 30 for focusing the electron beam 14 on the CRT's display screen, or faceplate, 24 shown in FIG. 2. Disposed on the inner surface of the CRT's display screen 24 is a phosphor layer 26 which emits light in response to the electron beam 14 incident thereon.
An elevation view of the CRT's display screen 24 is shown in FIG. 3 which also illustrates the horizontal scan lines 25 over which the electron beam is displaced in tracing out a video image on the display screen. For simplicity, only 12 scan lines are shown in the figure, it being understood that there are many more horizontal scan lines in the typical CRT. The beginning of electron beam trace of the first horizontal scan line is shown by the arrow in the upper left-hand corner of FIG. 3, while the beginning of electron beam trace of the last horizontal scan line is shown by the arrow in dotted-line form in the lower left-hand corner of the figure. The electron beam is traced across the display screen 24 in a raster-like manner in proceeding from left to right and from top to bottom as viewed in FIG. 3. Each horizontal sweep by the electron beam of faceplate 24 provides a single horizontal line of the video image displayed thereon. Electron gun 10 is typical of those used in conventional monochrome CRTs which generally suffer from the design and operating limitations discussed above.
The present invention addresses the aforementioned limitations of the prior art by providing a multi-beam electron gun for a monochrome CRT which does not require costly electron beam magnetic focusing, high electron emission density cathode, or a high frequency magnetic deflection yoke.